Journal of Clinical Endocrinology & Metabolism, doi:10.1210/jc.2005-0920

High circulating Thyrotropin levels in obese women are reduced after body weight loss induced by caloric restriction
Petra Kok, Ferdinand Roelfsema, Janneke G. Langendonk, Marijke Frölich, Jacobus Burggraaf, A. Edo Meinders and Hanno Pijl

Context: Previous clinical studies concerning the impact of body weight loss on single plasma TSH concentration measurements or the TSH response to TRH in obese humans have shown variable results.

Objective: The objective of this study was to investigate the effect of weight loss induced by caloric restriction on diurnal TSH concentrations and secretion in obese humans.

Design: This was a clinical, prospective, crossover study.

Setting: The study was conducted at the Clinical Research Center of Leiden University Medical Center.

Participants: Eleven obese premenopausal women (body mass index, 33.3 ± 0.7 kg/m2) were studied.

Intervention: The study intervention was weight loss (50% reduction overweight by caloric restriction).

Main Outcome Measure(s): Twenty-four-hour plasma TSH concentrations (10-min intervals) and the 24-h TSH secretion rate, calculated by a waveform-independent decon¬volution technique (Pulse), were determined.

Results: The 24-h TSH secretion rate was significantly higher in obese women than in normal weight controls, and weight loss was accompanied by diminished TSH release (be¬fore weight loss, 43.4 ± 6.4 mU/liter•24 h; after weight loss, 34.4 ± 5.9 mU/liter•24 h; P = 0.02). Circulating free T3 levels decreased after weight loss from 4.3 ± 0.19 to 3.8 ± 0.14 pmol/liter (P = 0.04). Differences in 24-h TSH release correlated positively with the decline of circulating leptin (r2 = 0.62; P < 0.01).

Conclusions: “Elevated TSH secretion in obese women is significantly reduced by diet-induced weight loss. Among various physiological cues, leptin may be involved in this phenomenon. The decreases in TSH and free T3 may blunt energy expenditure in re¬sponse to long-term calorie restriction, thereby frustrating weight loss attempts of obese individuals.”

This study demonstrates that there is a significant reduction in thyroid levels with dieting that is not detected by standard thyroid testing and the use of standard normal ranges.